Iím trying to understand something about what we are talking about when we talk about pitch rates for yeast. I understand how to calculate a proper pitch rate (both in theory and how to do it), and I understand some of the factors that affect fermentation.

Here is my question: when we pitch ~180 billion yeast cells into a 1.50 wort, e.g., are we pitching the entire population of cells that will be responsible for producing appropriate fermentation (in terms of producing the flavors we want and not producing the flavors we donít)? Or, are we pitching a starter population that is large enough to reproduce at an appropriate rate so that ferment appropriately? In other words, is the sweet spot in pitching calculations meant to hit the final number of cells our wort needs, or the number of cells needed to produce that final number? A third, hypothetical, way to say it: if there were no reproduction of yeast past what was pitched (and if none died), would we have a beer with the yeast-driven characteristics (in both a positive and negative sense) that we wanted?

I had thought that our pitching number was the number needed for successful reproduction in the wort, which leads to successful fermentation, rather than the number needed for successful fermentation per se, but I may be misunderstanding.

As progmac said. The "lag time" is when the yeast are reproducing to the appropriate colony size to ferment the wort. That's what they need the oxygen for (aerobic reproduction). Once they get to work actually fermenting the beer, they no longer need/take up oxygen (that's why fermentation is described as anaerobic), and any additional oxygen will result in oxidation of the beer.

If there is sugar, yeast will reproduce. In the absence of oxygen (which is depleted very quickly) reproduction is slower, but still significant. The "Lag Time" is often mis represented. Yeast reproduce during the "lag" and also during fermentation. It's the Balling Observation: 2.0665g of fermentables -> 0.11g yeast during anaerobic respiration. During the lag you don't see much activity because the population is relatively small. During a normal fermentation yeast population will grow about ten fold. Only about 20% of that is during the lag phase.

My book has a better description of this. George Fix "Principles of Brewing Science" is also a very good read.

I don't see how a colony of fungus could know how big the colony needs to be.

It could sense the sugar levels, but not whether it is in a 5 or 10 gallon batch.

^^^ This is spot on.

It's not until the cell density is much higher than it typically gets when fermenting beer that there is any effect of crowding. (I've seen some effect at 300 billion per liter. The data is on my blog, but it's described and displayed much better in my book.)

I don't see how a colony of fungus could know how big the colony needs to be.

It could sense the sugar levels, but not whether it is in a 5 or 10 gallon batch.

Yeah, sometimes people personify yeast. I think its just a matter of what compounds are available.

The problem with small pitching rates is the population has a maximum number of divisions it can perform... I think it is like 12 or something. Additionally they get stressed every time they reproduce. Therefore if they don't multiply fast enough, you get this stressed out damaged yeast doing a bunch of messed up fermentation instead of a huge crowd of new healthy yeast doing it.